1. Field of the Invention
The present invention generally relates to devices for preventing sudden electrostatic discharge, and more particularly to a wrist strap having an integral grounding tether, the grounding tether having a resistive portion formed from a conductive polymeric material.
2. Description of the Prior Art
Electrostatic discharge, as well as the mere presence of a static electric field, can be extremely detrimental to sensitive electronic parts. This is particularly true of modern semiconductors and integrated circuits which may be degraded or destroyed by sudden electrical discharge or the presence of a static field in the workplace. Especially sensitive components can be severely affected by an electrical potential as small as 50 volts, yet the simple act of walking has been known to triboelectrically generate a potential of 30,000 volts or more.
The most common tool heretofore used in the battle against electrostatic discharge is a conductive tether which is designed to drain away excess electrostatic buildup. Such tethers are widely used with straps for attachment to the ankle, wrist and body garments. The primary disadvantage of these prior art straps is their relatively high expense. These devices are also bulky and difficult to store, as compared to the subject device. The present invention obviates these problems by providing a disposable wrist strap which is integrally formed with a grounding tether, greatly reducing manufacturing cost. While there are numerous variations of wrist straps and grounding tethers, the inventor is aware of only two patents which disclose an article similar to the present invention, viz., a wrist strap integrally formed with the grounding tether. Those references are U.S. Pat. Nos. 3,857,397 issued to A. Brosseau on Dec. 31, 1974, and U.S. Pat. No. 4,698,724 issued to R. Burvee on Oct. 6, 1987.
One danger in the use of grounding tethers is the possibility that the tether may accidentally come into contact with a high voltage source, or may inadvertently be connected to such a source rather than to earth ground. Practitioners in this area have realized that this hazard may be minimized by providing a resistor in series with the tether, usually on the order of one megohm. In this manner, the resistor limits the current flowing through the tether, sparing the user from a high voltage shock. It has, however, been difficult in the past to provide this feature without adding considerable expense to a disposable strap-tether system.
In the Burvee patent (FIG. 9), a discrete resistance is created in the conductive plastic of the tether by forming an extended area of decreased crosssection. This particular version of the Burvee invention is embodied in a product sold by Minnesota Mining and Manufacturing Co. ("3M"--assignee of the present invention) as the Model 2209 disposable grounding wrist strap. The area of decreased cross-section is constructed by heating a portion of the plastic and embossing it, which does not always result in consistent resistance values in the discrete resistance portion. It would, therefore, be desirable and advantageous to devise a grounding wrist strap having a discrete portion of predictable resistance value, the discrete resistance portion being formed in a single step.
Accordingly, the primary object of the present invention is to provide a grounding wrist strap.
Another object of the invention is to provide such a wrist strap which is formed integrally with a grounding tether, thereby reducing expense.
Still another object of the invention is to provide wrist strap and tether having a discrete resistance portion.
Yet another object of the invention is to provide a simplified method of creating such a wrist strap and tether.